Effect of Silicon Carbide on High Temperature Mechanical Properties of Silicon-Molybdenum Ductile Iron Exhaust Manifold

1 Overview
Our company is a foundry enterprise mainly engaged in the production of exhaust manifolds. The exhaust manifold is an important component in internal combustion engines. It is difficult to forcibly cool at high temperatures, and the working conditions are extremely harsh.
As the internal combustion engine continues to improve, the exhaust gas temperature gradually increases, and the exhaust manifold is made of the original gray cast iron, gradually to ductile iron, vermicular cast iron, silicon molybdenum vermicular cast iron, silicon molybdenum ductile iron, high nickel austenite. Ductile iron, heat resistant steel evolved. The adapted exhaust manifold material is also different depending on the exhaust gas temperature of the internal combustion engine. Figure 1 is a six-cylinder exhaust manifold of a silicon-molybdenum ductile iron. There are many isolated hot sections, it is difficult to make up, the tendency to shrink is large, the spheroidization rate is required to be more than 90%, and it is required to meet the working requirements of 780 °C.
The corresponding working temperatures of the materials are shown in Table 1.
2. Test topics
One of the exhaust manifolds produced by our company is made of high silicon molybdenum. The physical and chemical requirements are shown in Table 2.
The production process is 3t / h medium frequency induction furnace melting, using raw materials are Q12 pig iron, low manganese scrap steel, silicon molybdenum ductile iron back charge, ferromolybdenum, spheroidizing agent FeSiMg6RE2, inoculant FeSi75-B. The chemical composition and physical and chemical indicators of the obtained castings meet the requirements of Table 2, and the measured values ​​are shown in Table 3.
It can be seen from Table 3 that although the physical and chemical detection of castings is higher than the index value at room temperature, the high temperature mechanical properties of 780 °C are lower than the technical requirements, and the performance of the exhaust manifold is mainly reflected in the high temperature environment.
When the engine is under high load operation, the exhaust manifold is plastically deformed by the high temperature thermal expansion to generate compressive stress, and at low load, the lower tensile stress is locally formed due to the temperature decrease. This change in the thermal stress field constitutes the thermal fatigue stress, and when the thermal fatigue stress reaches a certain value, the exhaust manifold will fail. And during engine operation, when there is a resonance effect between the natural frequency of the exhaust manifold and the main frequency of the engine vibration, the service life of the exhaust manifold will be drastically reduced.
On the basis of unadjustable product structure and chemical composition, in order to improve the high-temperature performance of the exhaust manifold, process optimization is required during the smelting process.
3. Silicon Carbide Applications
Silicon carbide was discovered by American Acheson in the 1961 fused diamond experiment. After 1987, the silicon carbide production line was established with the research results of CREE. Silicon carbide was used in the industrial field. Initially, silicon carbide was mainly used in the abrasive, refractory and metallurgical industries, and was rarely used in the foundry industry. Later, it was found that adding a small amount of silicon carbide in the melting furnace has a good pre-incubation effect, which is beneficial to improving the metallurgical quality of the cast iron. There is almost no use of silicon carbide in foundries that use induction furnaces abroad.
At present, in China's foundry industry, the number of foundries using induction furnaces as cast iron smelting equipment is increasing, but there are few SiC used in the smelting process, which can make it play less fully, and silicon carbide is mainly used in gray cast iron. In the case of cast steel, the application on silicon-molybdenum ductile iron is still rarely reported in the literature.
Our company is also in the exploration stage in the use of silicon carbide.
(1) The role of silicon carbide According to the literature, silicon carbide is used as a deoxidizer in steelmaking. Compared with the original process, the physical and chemical properties are more stable, the deoxidation effect is better, the deoxidation time is shortened, energy is saved, and steelmaking efficiency is improved. It is of great value to improve the quality of steel, reduce the consumption of raw and auxiliary materials, reduce environmental pollution, improve working conditions, and improve the comprehensive economic benefits of electric furnaces. The application of silicon carbide in gray cast iron can increase the graphite core, make the flake graphite fine, increase the degree of graphitization, reduce the tendency of white mouth, and improve the mechanical properties. Silicon carbide is applied on silicon-molybdenum ductile iron. It is hoped to increase the graphite core, increase the number of eutectic groups, increase the number of graphite spheres, and reduce the degree of supercooling of molten iron. The reaction of SiC+FeO=Si+Fe+CO, SiC To reduce the content of FeO and MnO in the molten iron, purify the molten iron, and thereby achieve the purpose of improving the high temperature mechanical properties of the silicon-molybdenum ductile cast iron.
(2) Specification and dosage of silicon carbide At present, China's industrial production of silicon carbide is divided into black silicon carbide and green silicon carbide. The physical and chemical indicators are shown in Table 4.
The amount of silicon carbide added is between 0.8% and 2.3%. If it is too low, the effect on metallurgical quality is not obvious. If it is too high, it is hardly soluble in molten iron. The remaining silicon carbide is introduced into the casting with micro-particles. The amount of hour is good for strengthening the matrix, and when the amount is large, it affects the strength of the matrix. Therefore, the final amount of silicon carbide added is 0.8% to 1.0%.
(3) Use of silicon carbide The melting point of silicon carbide is 2700 ° C, which is not melted during the smelting process and can only be dissolved in molten iron. The reaction equation is:
SiC+Fe→FeSi+C (unbalanced graphite)
In the formula, Si in SiC is combined with Fe, and the remaining C is unbalanced graphite, which is the core of graphite precipitation. Unbalanced graphite causes C to be unevenly distributed in the molten iron, the local C element is too high, and the "carbon peak" appears in the microdomain. This new graphite has high activity, and its mismatch with carbon is zero, so it is easy to absorb carbon in the molten iron, which promotes the increase of the crystal core and the number of graphite balls is significantly increased (see Figures 2 and 3). The spheroidization rate is increased.
The dissolution time of silicon carbide decreases as the particle diameter increases, and the dissolution rate increases, but its dissolution rate decreases as its concentration in the molten iron increases. In the effect of carbon and silicon content on the dissolution rate of silicon carbide, the effect of carbon is greater than that of silicon.
The dissolution of silicon carbide in the molten iron is affected by the degree of stirring of the molten iron and the addition time. The better the degree of stirring of the molten iron, the earlier the addition time, the more the diffusion of silicon carbide in the molten iron is more sufficient, and the metallurgical quality is more obvious. .
When silicon carbide is used, the amount of impurities is first removed when calculating carbon and silicon, and then according to the molecular formula of silicon carbide.
Carbon increase: C=C/(C+Si)=12/(12+28)=30%
Silicon addition: Si=Si/(C+Si)=28/(12+28)=70%
The production process is unchanged except for the addition of 0.8% silicon carbide.
First add the pig iron to melt, add some scrap after adding the molten iron, reduce the carbon content of the iron, then add about 1/2 amount of silicon carbide, then feed the smelt, and then add the remaining iron solution to 2/3 The silicon carbide is prevented from being dissolved due to premature addition and condensation on the bottom of the furnace, shortening the dissolution time of the silicon carbide, and fully utilizing the stirring action of the molten iron to fully diffuse the silicon carbide.
4. Comparison of effects
The test castings were tested according to the physical and chemical technical requirements of Table 2. The results are shown in Table 5.
From the comparison results of Table 5 and Table 3, it can be seen that there is no significant difference between the chemical composition and the matrix structure of the two; the number of graphite spheres is greatly improved due to the increase of the crystal core; the mechanical properties at room temperature are not much different; Silicon carbide is superior to the original process, and has increased by about 20% on the basis of the original process, which has greatly improved the high-temperature mechanical properties of the casting and achieved the intended purpose.
5 Conclusion
(1) In the smelting process of silicon-silicon molybdenum ductile iron, silicon carbide has good carbonation and silicon-enriching effect. After spheroidization and inoculation treatment, the carbon and silicon contents in the molten iron will not change abnormally.
(2) Whether or not silicon carbide is used, the microstructure of the silicon-molybdenum ductile iron does not change much, but the graphite size becomes smaller and the number increases.
(3) The use of silicon carbide has no significant effect on the mechanical properties at room temperature of silicon-molybdenum ductile iron.
(4) The use of silicon carbide can significantly improve the high temperature mechanical properties of silicon-molybdenum ductile iron, especially the increase in yield strength, which is beneficial to improve the high temperature fatigue performance of the exhaust manifold under cold and heat alternating.
About the author: Ren Leopard, Zhang Yi, Chai Xicheng, Zhao Xinwu, Xixia County Internal Combustion Engine Exhaust Pipe Co., Ltd.

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